"670-km discontinuity","Boundary that separates the upper mantle (35-670 km) from the lower mantle (670-2900 km). It could be a natural barrier to whole-mantle convection.	","","","","","",""
"A-type earthquake","Events with clear P-waves and S-waves occurring under volcanoes at depths of 1-10 km.","","","","","",""
"aa","Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments. Aa flows commonly develop from pahoehoe flows as they cool and lose gas.","","","","","",""
"ablation (glacial)","All processes which include melting  evaporation (sublimation)  wind erosion and calving (breaking off of ice masses)  that remove snow or ice from a glacier or snowfield.  The term also refers to the amount of snow orice removed by these processes.
","","","","","",""
"absolute date","An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.","","","","","",""
"abyssal plain","The broad expanse of sea floor lying between about 3 and 6 kilometers (-2 to 4 miles) below sea level.","","","","","",""
"accelerometer","A seismograph for measuring ground acceleration as a function of time.","","","","","",""
"accretionary wedge","Accumulation on the ocean floor of sedimentary material scraped off the subducted plate by the upper plate.","","","","","",""
"accumulation (glacial)","All processes including snowfall condensation  avalanching  snow transport by wind and freezing of liquid water that add snow or ice to a glacier floating ice or snow cover. The term also includes the amount of snow or other solid precipitation added to a glacier or snowfield by these processes.","","","","","",""
"accumulative minerals","Minerals that make up the extract assemblage during fractional crystallization of a magma.","","","","","",""
"active fault","A fault along which slip has occurred in historical (or Holocene) time or earthquake foci are located.","","","","","",""
"active lobe"," of a delta. The site on a delta where functioning distributary channels cause the delta to grow seaward.","","","","","",""
"active volcano","A volcano that is erupting; or one that while not erupting at the present  has erupted within geologically recent time and is considered likely to do so in the geologically near future.","","","","","",""
"actualism","The  interpretation  of ancient rocks by applying the results of analyses of modern-day geologic processes in accordance with the principle of uniformitarianism.","","","","","",""
"adaptation","A feature of an organism that serves one or more functions useful to the organism.","","","","","",""
"adaptive breakthrough","An evolutionary innovation that affords a group of organisms a special ecologic opportunity and often leads to the adaptive radiation of that group.","","","","","",""
"adaptive radiation","Evolutionary diversification of a taxon's morphology, ecology,  physiology, behavior and other characteristics over a geologically short time interval  leading to the appearance of a number of new taxa.","","","","","",""
"adaptive radiation","The rapid origins of many new species or higher taxa from a single ancestral group.","","","","","",""
"aeolian deposits","Sedimentary deposits consisting of windblown dust.","","","","","",""
"AFMAG","Audio Frequency Magnetic technique  in which natural electomagnetic fields in the audio-frequency range are used to study lateral changes in earth resistivity.","","","","","",""
"aftershock","An earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks  decreasing in frequency with time.","","","","","",""
"age"," geologic. The division of geologic time smaller than an epoch.","","","","","",""
"aggregation","The process of combination of very small volcanic ash or dust particles in volcanic plumes in the atmosphere to form larger particles which settle out rapidly to the earth's surface due to their large diameter.","","","","","",""
"Agung","Active volcano,  elevation 3141 m (10380 ft)"," in Bali,  Indonesia (8.342ø S 115.508ø E).  Agung is known to have erupted three times,"," the last in 1964.","","","",""
"albedo","The percentage of solar radiation reflected from the earth's surface. This percentage is higher for ice than for land or water and usually higher for land than for water.","","","","","",""
"algal ridge","The  durable  structure formed by coralline algae that buttresses the front of a modern coral reef.","","","","","",""
"alkane","One of the group of hydrocarbons of the paraffin series,  for example  methane,  ethane  and propane.","","","","","",""
"allelochemicals","Chemicals produced by one soecies that enter the environment and influence other species.","","","","","",""
"allochthonous clay","Clay originating in an environment different from the final sedimentary environment (rocky substrate  soil  or upstream sediment subjected to erosion).","","","","","",""
"alluvium","Unconsolidated detrital material deposited during comparatively recent geologic time by a stream or river. Alluvial deposits include mid channel bars,  point bars,  floodplain deposits,","alluvial fans,  and fluvial terraces.","","","","",""
"alluvlal fan","A low cone-shaped structure that forms where an abrupt reduction in slope -- for example,  the transition from a highland area to a broad valley -- causes a stream to slow down.","","","","","",""
"amniote egg","The type of egg laid by reptiles and birds  having a nutritious yolk and a hard outer shell to protect the embryo from the dry environment. The amniote egg is named for the amnion a sac that contains the embryo.","","","","","",""
"amplitude","The maximum height of a wave crest or depth of a trough.","","","","","",""
"AMT","Audio MagnetoTelluric method  in which magnetotelluric measurements are made in the audio frequency range (10-10,000 Hz) for medium to shallow depth investigation.","","","","","",""
"andesite"," A find-grained extrusive igneous rock intermediate in composition between rhyolite which is felsic and basalt which is mafic.","","","","","",""
"andesite","Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium. Andesite minerals commonly include plagioclase and hornblende with lesser amounts of mica, pyroxene and various accessory minerals. Andesites are aphanitic in texture and are usually medium dark in color. They occur with composite volcanic cones associated with convergent plate margins.","","","","","",""
"angular unconformity","An unconformity separating horiiontal strata above from older strata that had been tilted and eroded.","","","","","",""
"anhydrite","The mineral that consists of calcium sulfate (CaS04)  or the rock composed of this mineral.","","","","","",""
"anomaly"," magnetic. A local increase or decrease in the strength of the earth's magnetic field caused by the magnetism of nearby sediments or rocks.","","","","","",""
"anticline","A fold that is concave in a downward direction -- that is the vertex is the lowest point.","","","","","",""
"aquiclude","An impermeable geologic formation or stratum which will not hold or transmit fluid.","","","","","",""
"aquifer","Saturated permeable geologic unit that can store and transmit significant quantities of groundwater.","","","","","",""
"aquitard","A geologic formation or stratum that significantly retards fluid movement.","","","","","",""
"arc tholeiite","Basalt  with a characteristic chemical composition found in an island or continental volcanic arc (also a high-K tholeiite).","","","","","",""
"array","An ordered arrangement of seismometers or geophones the data from which feeds into a central receiver.","","","","","",""
"arrival","The appearance of seismic energy on a seismic record.","","","","","",""
"arrival time","The time at which a particular wave phase arrives at a detector.","","","","","",""
"artesian well","A well in an aquifer where the groundwater is confined under pressure and the water level will rise above the top of the confined aquifer.","","","","","",""
"artificial recharge","The unnatural addition of surface waters to groundwater. Recharge could result from reservoirs  storage basins,  leaky canals,"," direct injection of water into an aquifer,"," or by spreading water over a large land surface.","","","",""
"aseismic","Not associated with an earthquake  as in aseismic slip. Also used to indicate an area with no record of earthquakes; an aseismic zone.","","","","","",""
"ash","Fine particles of rock material ejected during an explosive volcanic eruption (commonly intermediate to felsic events). Ash may be either solid or molten when first erupted  and generally measures less than 0.10 inch in size (larger particles have other names).","","","","","",""
"ash flow","A turbulent mixture of gas and rock fragments most of which are ash-sized particles  ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes"," or even along a level surface.","","","","",""
"ashfall (subaerial)","Volcanic ash that has fallen through the air. The resulting deposit is usually well sorted and exhibits a finely layered structure.","","","","","",""
"asthenosphere","The layer below the lithosphere which is marked by low seismic wave velocities and high seismic wave attenuation and probably partially molten.  The layer or shell of Earth below the lithosphere which is weak and in which large-scale isostatic adjustments take place, magmas may be generated,and seismic waves are strongly attenuated.","","","","","",""
"atoll","A circular or horseshoe-shaped organic reef growing on a submerged volcano.","","","","","",""
"authigenesis","Process by which new minerals form in place within a sediment during deposition or in the early stages of diagenesis.","","","","","",""
"autochthonous clay","Clay originating in the soft sediment or sedimentary rock where it is identified and where it formed either by neoformation (i.e., from dissolved elements) or by transformation (i.e., from mineral growth on preexisting sedimentary substrate).","","","","","",""
"avalanche","A large mass of material falling or sliding rapidly due to the force of gravity. In many cases, water acts as a catalyst and/or lubricant. Avalanches often are classified by what is moving, such as a snow,  ice, soil, or rock avalanche. A mixture of these materials is commonly called a debris flow.","","","","","",""
"axial plane","An imaginary plane that cuts through a fold"," dividing it as symmetrically as possible.","","","","",""
"B-type earthquake","Events with weak P-waves and no S-waves"," a low-frequency content  and occurring under volcanoes at depths < 1 km.","","","","",""
"banded iron formation","An iron formation that consists of alternating iron-rich and iron-poor layers. Most rocks of this type are older than about 2 billion years.","","","","","",""
"barrier island","An elongate island composed of sand heaped up by waves that lies approximately parallel to the shoreline of an ocean.","","","","","",""
"barrier reef","An elongate organic reef that parallels a coastline and is large enough to dissipate ocean waves, leaving a quiet-water lagoon on its landward side.","","","","","",""
"basal sliding","The movement or speed of movement of a glacier on its bed.  Also called basal slip.","","","","","",""
"basalt","Volcanic rock (or magma) that is generally dark in color, contains 45 to 54 percent silica, and is rich in iron and magnesium.  An eruption of basaltic magma is generally quiet and results in flows (both vesicular and non-vesicular) and breccias. Undersea eruptions commonly result in the formation of "pillow lavas." Basalt represents the initial differentiated material erupted by the earth at spreading centers.","","","","","",""
"basement rocks","Rocks beneath a large geologic feature (such as the fold-and-thrust belt of a mountain system) which are genetically unrelated to the overlying feature.","","","","","",""
"bed","A distinct sedimentary layer (stratum) thicker than 1 centimeter.","","","","","",""
"belemnite","Extinct invertebrate animal (cephalopod) known from cigar-shaped fossils.","","","","","",""
"Benioff zone","A narrow zone defined by earthquake foci and tens of kilometers thick dipping from the surface under the Earth's crust. Zone of earthquake foci produced near the surface of and within the subducted lithosphere.","","","","","",""
"benthic foraminifera","Benthic protozoans (i.e.,  live on the seafloor)","which may form carbonate tests. The tests are preserved in sediments and form an important basis for paleoceanographic reconstructions.","","","","",""
"bentonite","A clay material composed principally of the mineral montmorillonite. It has a great affinity for fresh water and when hydrated will increase its volume more than seven times. Water/bentoninte suspensions are essentially impermeable. Commonly used as a sealant for ponds.","","","","","",""
"bimodal distribution","A size or mass spectrum that has two peaks of concentration.","","","","","",""
"biogeochemistry","The chemical interactions among the atmosphere, biosphere, hydrosphere","and lithosphere. Study of the cycling transformations  and transport of chemicals in and between landscapes and ecosystems.","","","","",""
"biogeography","The study of the distribution of organisms on a geographic scale.","","","","","",""
"biostratigraphic unlt","A body of rock such as a zone  defined on the basis of its fossil content and having approximately time-parallel upper and lower boundaries.","","","","","",""
"biostratigraphy","Relative (not absolute,  i.e.  number of years) age dating and time correlation of stratified rocks (mostly sedimentary plus some volcanics) by means of fossils contained in those rocks based on our knowledge of their time sequence locally regionally and globally -- all of this ultimately being a function of evolutionary change.","","","","","",""
"biostratigraphy","The study and classification of rocks and their history based on their fossil content.","","","","","",""
"bioturbation","Disturbance of oceanic sediments near the water interface by bottom-dwelling (benthic), organisms, for example, burrowing worms.","","","","","",""
"bird's-eye limestone","Layered limestone that is full of holes  some of which may have been secondarily filled by cementing minerals.  The holes were produced by burrowing animals or by gas bubbles. Most of these limestones are of intertidal or supratidal origin.","","","","","",""
"black smokers","Seafloor hydrothermal fluids jetting from chimneys typically about 10 cm in radius. Flow rates are ~ 1-5 m/s; temperature is 350ø-400ø C. Black coloration is due to sulfide mineral precipitates.","","","","","",""
"body wave","A seismic wave that can travel through the interior of the earth.  P-waves and S-waves are body waves.","","","","","",""
"bomb","Fragments of molten or semi-molten rock  several inches to several feet in diameter which are blown out during an explosive volcanic eruption. Because of their semi-plastic condition bombs are often modified in shape during their flight or upon impact.","","","","","",""
"borehole  geophysics","Science of (seismic or electrical) physical measurement between boreholes or between a borehole and the surface.","","","","","",""
"boulder","A piece of gravel larger than 256 millimeters (~10 inches) in size.","","","","","",""
"brackish water","Water that is lower in salinity than normal sea water and higher in salinity than freshwater"," ranging from 30 to 0.5 parts salt per 1000 parts water.","","","","",""
"braided stream","A stream that has many intertwining channels separated by bars of coarse sediment. Braided streams develop where sediment is supplied to the stream system at a very high rate on an alluvial fan,  for example, or in front of a melting glacier.","","","","","",""
"breccia","A rock that resembles conglomerate in consisting of clasts of gravel surrounded by sand, but in breccia the clasts are angular, whereas in conglomerate they are rounded.","","","","","",""
"brittle-ductile transition zone","The location at depth within the earth's crust where the temperature and pressure have risen to such a high level that directed stress results in plastic deformation as opposed to fracturing and faulting.","","","","","",""
"Brownian coagulation","The process by which a particle grows by the rapid condensation of gases and fine particles.","","","","","",""
"calcrete","A surficial gravel and sand conglomerate cemented by calcium carbonate.","","","","","",""
"caldera","From the Spanish for cauldron.  A basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.","","","","","",""
"caliche","Nodular calcium carbonate (opaque  reddish-brown-to-white) that accumulates in the B-horizon ot soils in warm climates that are dry part of the year. Also called calcrete.","","","","","",""
"carbonaceous chondrite","A stony meteorite that contains carbon compounds.","","","","","",""
"carbonate mineral","A mineral in which the basic building block is a carbon atom linked to three oxygen atoms. Calcite, aragonite and dolomite are the most abundant examples found in sediments and sedimentary rocks.","","","","","",""
"carbonate rock","A sedimentary rock that consists primarily of carbonate minerals. The dominant mineral is nearly always either calcite, in which case the rock is limestone,  or dolomite,  in which case the rock is dolomite.","","","","","",""
"carbonate rocks","General term for limestones and dolostones.","","","","","",""
"carbonate sediment","Unconsolidated sediment that consisls primarily of carbonate minerals, usually aragonite or calcite.","","","","","",""
"carbonization","The mode of fossilization in which liquids and gases escape,  leaving a residue of carbon on the surface of an impression of the organism.","","","","","",""
"cataclastic rock","Metamorphic rocks that resemble breccias or poorly sorted sandstones and that form by dynamic metamorphism, which breaks and reorients grains.","","","","","",""
"catastrophism","The doctrine that sudden violent and widespread events caused by supernatural forces formed most of the rocks that are visible at the earth's surface.","","","","","",""
"catena","Sequence of proximately located soils of about the same age and derived from similar parent material but having different characteristics due to variation in relief and/or drainage.","","","","","",""
"cation exchange capacity","Sum total of exchangeable cations retained by negatively charged soil particles. Units of milliequivalents per 100 grams of soil. Abbreviated CEC.","","","","","",""
"cementation","The lithification of sediment by the precipitation of minerals from watery solutions percolating through the sediment.","","","","","",""
"central angle","An angle with the vertex at the center of the Earth with one ray passing through the hypocenter (and also the epicenter) and the other ray passing through the recording station.","","","","","",""
"chemical remanent magnetization","Magnetization acquired isothermally through chemical change at temperatures below the Curie temperature either by grain growth or by alteration of a magnetic parent to a magnetic daaghter.","","","","","",""
"chemical sediment","A sediment created by precipitation of one or more minerals from natural waters.","","","","","",""
"chemosynthesis","The breakdown of simple chemical compounds within a cell for the production of energy. Sulfate-reducing bacleria exemplify this process.","","","","","",""
"chert","An impure rock, often gray in color, that consists primarily of extremely small quartz crystals precipitated from water solutions.","","","","","",""
"chloroplast","A body within a plant cell or plantlike cell that serves as the site of photosynthesis within the cell. Chloroplasts are apparently evolutionary descendants of blue-green algae that became trapped in other single-celled organisms.","","","","","",""
"cinder cone","A volcanic cone built entirely of loose fragmented material (pyroclastics).","","","","","",""
"circumpolar current","The circular flow of water around Antarctica"," resulting from the juncture of the west-wind drifls of the Atlantic Pacific and Indian Oceans.","","","","",""
"clast","A solid product of erosion. Clasts are sometimes referred to as detritus or detrital material.","","","","","",""
"clastic","Pertaining to a rock or sediment composed principally of fragmental detrital materials.	","","","","","",""
"clastic rock","A rock that is an aggregate of detrital material"," or clasts.","","","","",""
"clay","Particles of size less than two microns basically made up of clay minerals which are hydrated aluminosilicates.	","","","","","",""
"clay-sized sediment","Sediment in which particles are smaller than 1/256 millimeter. Most sediment of this size belongs to the clay mineral family.","","","","","",""
"claystone","A sedimentary rock that consists primarily of clay but that is not fissile like shale.","","","","","",""
"closed hydrochemical system","Physical portion of the earth in which the mass of a given immobile chemical element remains constant throughout a chemical and physical transformation process and is enriched only by removal of mobile elements.","","","","","",""
"coal","A metamorphic rock formed from stratified plant remains. It contains more than 50 percent carbon and burns readily.","","","","","",""
"cobble","A piece of gravel between 8 and 256 millimeters in size.","","","","","",""
"coccolith","One of the many armorlike plates that surround single-celled marine algae known as coccolithophores.","","","","","",""
"cogenetic (comagmatic)","Igneous rocks related by differenntiation to a common parental source.	","","","","","",""
"components principle","The principle stating that a body of rock is younger than any other body of rock from which any of its components are derived.","","","","","",""
"composite volcano","A steep volcanic cone built by both lava flows and pyroclastic eruptions.","","","","","",""
"compositional maturity","Extent to which a clastic sediment approaches the compositional end product of chemical weathering; the extent to which the most stable components of a sediment have been enriched over unstable components. In sandstone,  compositional maturity can be conveniently expressed in terms of the abundance of quartz  the most stable common phase in most sandstones.","","","","","",""
"concretion","A hard nodular structure formed in sediment or in a sedimentary rock by diagenesis.","","","","","",""
"condensation","The change of state of water from the vapor to the liquid phase. Results in liberation of 80 calories per cubic centimeter.","","","","","",""
"conduction","Transfer of heat from one place to another by molecular agitation and without movement of the medium.","","","","","",""
"conglomerate","A rock consisting of rounded clasts of gravel surrounded by sand.","","","","","",""
"connate water","Water included in the groundwater which is derived from the rock itself, as opposed to water which has percolated down from the surface.","","","","","",""
"contact metamorphism","Local meta morphism caused by igneous intrusion that "bakes" nearby rocks.","","","","","",""
"continental accretion","The marginal growth of a continent along a subduction zone by mountain building or by addition of a microplate.","","","","","",""
"continental crust","The layer of the Earth that lies under continents and the continental shelves. It ranges in thickness from 35 to 60 km. Its upper layer has a density of ~2.7 g/cm3 and is composed of rocks that are rich in silica and alumina.","","","","","",""
"continental crust","Solid  outer layers of the earth, including the rocks of the continents.","","","","","",""
"continental drift","The theory that horizontal movement of the earth's surface causes slow relative movements of the continents toward or away from one another.","","","","","",""
"continental margin","The submarine edge of the continental shelf,  from which the continental slope descends. In the modern world this margin generally lies about 200 meters (~600 feet) below sea level.","","","","","",""
"continental rise","A more gently sloping region along the base of the continental slope. The continental rise is a depositional feature formed of sediment transported down the slope often by turbidity currents.","","","","","",""
"continental shelf","That part of the continent that is covered by water.  Continental shelves are more than 322 km wide and about 122 m deep. At the edges of the shelves the continental slopes drop rapidly from 100 to 200 m to 3000 to 3700 m.","","","","","",""
"convection","Rotational flow of a fluid resulting from imbalances in density. This often occurs because the fluid below is heated and becomes less dense than the fluid above or because the fluid above is cooled and becomes more dense than the fluid below.","","","","","",""
"core","The innermost layers of the Earth. The inner core is solid and has a radius of about 1300 kilometers. (The radius of the Earth is about 6371 kilometers.) The outer core is fluid and is about 2300 kilometers thick. S-waves cannot travel through the outer core.","","","","","",""
"cosmogenic isotope","Isotope produced by nuclear reactions induced by cosmic-ray particles.","","","","","",""
"cotectic","Line  curve or surface on a phase diagram depicting crystal-melt equilibria along which two crytals co-precipitate.","","","","","",""
"crater","A steep-sided usually circular depression formed by either explosion or collapse at a volcanic vent or surface impact of a meteorite, asteroid, or comet.","","","","","",""
"crust","The outermost rock shell. Outer part of the earth composed essentially of crystalline rocks. Crustal thickness is approximately 50 km and outer radius is approximately 6371 km.","","","","","",""
"CSAMT","Controled Source Audio-MagnetoTelluric method in which an artificial source generally a long cable is used to simulate the audio-frequency magnetotelluric fields.","","","","","",""
"Curie temperature","Temperature above which there is no spontaneous magnetization.","","","","","",""
"dacite","Volcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium.","","","","","",""
"debris avalanche","A flow of unsorted masses of rock and other material downslope under the influence of gravity. Water is commonly involved as a catalyst and/or lubricant. For example, a rapid mass movement that included fragmented cold and hot volcanic rock,  water,  snow,  glacial ice, trees and other debris and hot pyroclastic material was associated with the May 18","1980 eruption of Mt. St. Helens. Most of the deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche resulting from the eruption.","","","","",""
"dendritic crystals","Crystals that grow in a branching form resembling trees.","","","","","",""
"dendrogeomorphology","Application of tree-ring studies (dendrochronology) to the interpretation of geomorphologic processes. Often supplemented by related botanical observations.","","","","","",""
"density","The mass per unit volume of a substance"," commonly expressed in grams per cubic centimeter.","","","","",""
"detachment plane","The surface along which a landslide disconnects from its original position.","","","","","",""
"detritus","Loose rock or mineral material that is dislodged from bedrock by mechanical means and transported from its place of origin.","","","","","",""
"dew point","The temperature (elevation) where adiabatic cooling results in the initiation of condensation of water vapor into cloud droplets.","","","","","",""
"di-polar","The arrangement of the hydrogen atoms of a water molecule at 105 deg. across the oxygen results in a slight electrical charge to the molecule.","","","","","",""
"diagenesis","Chemical and physical changes undergone by a sediment after its initial deposition,  exclusive of surficial alteration (chemical weathering) and metamorphism (pervasive recrystallization).  Includes processes like compaction, cementation"," and replacement.","","","","",""
"differentiation (fractionation)","Process by which one magma gives rise to another of different composition. Differentiated or fractionated magmas are said to be evolved from a more primitive parent. Processes by which magmas evolve from basaltic to more silicic compositions; includes crystal fractionation,  magma mixing,  and crustal contamination.","","","","","",""
"diffusion of magnetic field lines","Motion of magnetic field lines through an electrically conducting fluid by virtue of the fluid's finite electrical resistivity.","","","","","",""
"dilatancy","Inelastic volume increase caused by the occurrence of small cracks in rock or soil under stress.","","","","","",""
"dip","The angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane perpendicular to the strike.","","","","","",""
"dip-slip fault","A fault in which the relative displacement is along the direction of dip of the fault plane; the offset is either normal or reverse.","","","","","",""
"discharge area","Region in which there is a net loss of water from the groundwater system (discharge) to surface water bodies or by evapotranspiration.","","","","","",""
"dispersion (wave)","The spreading out of a wave train due to each wavelength traveling with its own velocity.","","","","","",""
"displacement","Vector defining offset on a fault that may represent slip in a single event or the accumulation of slip over a much longer time.","","","","","",""
"dissolved load","Part of the total stream load consisting of substances in dissolved form such as Ca2+,"," HCO-3,  H4SiO4.","","","","",""
"dome","A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent often circular in plane view and spiny,  rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler,  outer crust during growth of the dome.","","","","","",""
"dormant volcano","A volcano that  is presently inactive but which may erupt again. The major volcanic cones of the Cascade Range in Washington, Oregon, and California, for example, are believed to be dormant rather than extinct.","","","","","",""
"drift (glacial)","General term for material deposited by a glacier.","","","","","",""
"duricrust","Indurated soil crust or hardcap occurring on or near an eroded planation surface. May be indicative of climatic change from tropical to drier conditions with alternating wet and dry seasons. Contains tubular voids acting as avenues of translocation.","","","","","",""
"dust veil index","A quantitative method developed by H. H. Lamb for comparing the magnitude of volcanic eruptions. The formulae use observations either of the depletion of the solar beam, temperature lowering in the middle latitudes,  or the quantity of solid matter dispersed as dust. The reference dust veil index is 1000,  assigned to the Krakatoa 1883 eruption and the index is calculated using all three methods,  where the information is available for statistical comparison purposes. Abbreviated D.V.I.","","","","","",""
"dynamo action","Process whereby motions within an electrcally conducting fluid, interacting with a magnetic field, convert their kinetic energy into magnetic energy, thereby sustaining the magnetic field.","","","","","",""
"earthquake","The vibration of the Earth caused by the passage of seismic waves radiating from some source of elastic energy.","","","","","",""
"earthquake swarm","A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.","","","","","",""
"eccentricity","Measures how elliptical the earth-sun orbit is,","with consequent effects on solar radiation received by the earth. If e is the eccentricity"," then (1 + e)/(1 - e) is the ratio of the farthest and closest earth-sun distance.","","","",""
"ecosystem function","Ecological processes such as primary production,"," decomposition, and nutrient cycling that occur at the scale of a system and are affected by a variety of organisms and the environment's microclimate.","","","","",""
"effective diffusion coefficient","Measure of the rate of diffusion of a substance in a heterogeneous medium like a sediment-water system. Tortuosity resulting from the presence of solid particles is accounted for in this type of coefficient.","","","","","",""
"ejecta","Material that is thrown out by a volcano,  including pyroclastic material (tephra) and,"," from some volcanoes.  lava bombs.","","","","",""
"ejecta blanket","Circular apron of debris surrounding an impact crater.","","","","","",""
"El Chichon","Active volcano in Mexico (17.360ø N,","93.228ø W),  elevation 2225 m (7300 ft).  El Chichon is known to have erupted five times, most recently in 1983.","","","","",""
"elastic rebound theory","The theory of earthquake generation proposing that faults remain locked while strain energy slowly accumulates in the surrounding rock","and then suddenly slip,  releasing energy.","","","","",""
"elastic wave","A wave that is propagated by some kind of elastic deformation,  that is, a change in shape that disappears when the forces are removed. A seismic wave is a type of elastic wave.","","","","","",""
"electronegativity","Relative measure (on a scale of 4) of an atom's attraction for the electrons in its outer shell.","","","","","",""
"enrichment factor","Ratio of chemical concentration of an element in a weathered material to that in its fresh parent material.","","","","","",""
"epicenter","The point on the Earth's surface directly above the focus (or hypocenter) of an earthquake.","","","","","",""
"equilibrium line","The level on a glacier where accumulation equals ablation and the net balance equals zero.","","","","","",""
"eruption column","A mixture of hot volcanic ash,  gases and entrained atmospheric air that rises vertically above an erupting volcano to heights of several tens of kilometers in the earth's atmosphere. The rise of the column is in part due to kinetic energy of the jet of gases and ash issuing from the volcano,  but primarily due to the buoyant rise of the low-density mixture in the cooler atmosphere.","","","","","",""
"eruption plume","Dispersla of an eruption column downwind from the volcano, due to the effects of prevailing atmospheric circulation in the atratosphere and troposphere.","","","","","",""
"Euler pole","Pole passing through the center of the earth about which a plate can rotate as a rigid body from any initial to any final position on the earth's surface.	","","","","","",""
"evapotranspiration","Water used by plants and animals and subsequently returned directly to the atmosphere.","","","","","",""
"evergreen","Coniferous, needle-leaved or sclerophyllous plant that does not echibit a marked seasonal leaf fall.","","","","","",""
"exogenic cycle","Portion of the earth's system involved in the movement and mass transfer of substances through the cycle of weatherin,  erosion,","transportation, deposition, burial,","diagenesis and uplift. Reservoirs in the cycle generally include the atmosphere, ocean"," biota,  sediments and shallow crust.","","",""
"exogenic system","Earth's interacting envelopes: the hydrosphere (mostly the ocean), atmosphere, biosphere,","reactive lithosphere (that part of the rocky crust within reach of the processes of weathering). The processes of deep seawater circulation at spreading ridges overlap with endogenic (within the earth) processes. Also relevant are extraterrestrial processes. Refractory problems such as mass extinction usually become arenas for competing theories drawn from the three sources.","","","","",""
"explosion earthquake","Events with ground waves (including P-waves and S-waves) and often air waves, which travel through the air and are transmitted back into the ground in the vicinity of the seismometer.","","","","","",""
"extinct volcano","A volcano that is not presently erupting and is not likely to do so for a very long time in the future.","","","","","",""
"extract assemblage","Assemblage of accumulative minerals extracted from a parent magma duting fractional crystallization to produce a daughter magma.","","","","","",""
"fallout","Settling and deposition of particulate matter"," such as tephra and volcanic aerosols  out of an eruption plume and onto the earth's surface.","","","","",""
"fault","A fracture or zone of fractures in rock along which the two sides have been displaced relative to each other parallel to the fracture. The total fault offset may range from centimeters to kilometers. Large fracture in the ground with two sides displaced relative to each other in drections parallel to the fracture.	","","","","","",""
"fault plane","The plane that most closely coincides with the rupture surface of a fault.	","","","","","",""
"FEM","Frequency Electromagnetic Method, in which the primary field is sinusoidal in character.	","","","","","",""
"ferro-magnesian elements","Elements that are readily substituted into silicate minerals that have Fe and Mg as major constituents. Examples include Cr,  Ni, V, Sc, Co.","","","","","",""
"firn","Material that is transitional between snow and glacier ice. It is formed from snow after passing through one summer melt season and becomes glacier ice after its permeability to liquid water falls to zero.	","","","","","",""
"fissures","Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows,but pyroclastics may also be ejected.","","","","","",""
"flood basalt","Laterally extensive deposits of basaltic lava flows resulting from outpouring of vast volumes of magmas during fissure eruptions.	","","","","","",""
"flow rheology","Deformational character of the flow when stress is applied. Rheology varies due to differences in fluid strength, viscosity, density and sediment concentration.","","","","","",""
"fluid inclusion","Bubble of liquid or gas trapped inside a solid mineral-phase, usually formed during crystallization of the mineral in the presence of a fluid phase. Tiny cavities, 1.0-100 microns in diameter,  containing liquid and/or gas,","formed by the entrapment of fluid in crystal irregularities. Fluid inclusions provide information on the temperature, pressure,","and chemical composition of the fluids and/or gases trapped at the time of mineral formation.","","","",""
"fluorescence","Emission of a certain wavelength light"," resulting from and occurring only during the absorption of light of another wavelength.	","","","","",""
"foreshocks","Smaller earthquakes preceding the largest earthquake of a series concentrated in a restricted crustal volume.	","","","","","",""
"fumarole","A vent or opening through which issue steam, hydrogen sulfide  or other gases. The craters of many dormant volcanoes contain active fumaroles.","","","","","",""
"fumarolic emission","Low-level and generally steady output of low-temperature gases such as sulfur, halogens,","water vapor and carbon dioxide from dormant volcanoes and geothermal fields in volcanic regions.","","","","",""
"galvanic interaction","Redox reaction between two mineral entities with different rest potential, where the mineral with lower rest potential acts as reductant and the mineral with the higher rest potential acts as oxidant/reductant in conveying electrons to oxygen or some other suitable acceptor.	","","","","","",""
"gas-to-particle reaction","The formation of particles by the rapid condensation of a gas.	","","","","","",""
"Gaussian coefficients","Coefficients of a spherical harmonic expansion of the potential function of the geomagnetic field.	","","","","","",""
"geodetic","Pertaining to measurement of the shape and dimensions of the earth.	","","","","","",""
"geodimeter","A surveying instrument to measure the distance between two points on the Earth's surface.	","","","","","",""
"geoid","Level (or equipotential) surface at mean sea level. Surface of constant gravitational potential that is chosen to define the earth's shape. At sea the geoid corresponds to the time-averaged surface of the ocean (to an accuracy of 1-2 m).	","","","","","",""
"geomagnetic field","Main magnetic field of the earth -- the part generated within the earth.	","","","","","",""
"geomagnetic polarity time scale","Reference for the polarity history of the geomagnetic field, including current correlations of polarity intervals to biozonations,  geologic stages and numerical ages.	","","","","","",""
"geomorphology","The study of present-day landforms, including their classification,  description,  nature,  origin"," development,  and relationships to underlying structures. Also the history of geologic changes as recorded by these surface features. The term is sometimes restricted to features produced only by erosion and deposition.	","","","","",""
"geophysical noise","Fluctuations in a geophysical parameter that are of a quasirandom nature and cannot be attributed to fluctuations in known causative phenomena.	","","","","","",""
"geopotential model","Set of coefficients of a series representation of the earth's gravitational field in terms of spherical harmonics.	","","","","","",""
"geopressured systems","Hydrothermal systems contained in sedimentary rock in regions of normal geothermal gradient. They represent a special case in which the pore fluids are under pressure exceeding the hydrostatic pressure.	","","","","","",""
"geothermal power","Power generated by using the heat energy of the earth.","","","","","",""
"glacial abrasion","A common mechanical weathering process where rock and debris frozen into the sides and bottom of a glacier act like sandpaper and wear down the bedrock the glacier is mowing across.","","","","","",""
"glacial maximum","The position or time of the greatest advance of a glacier (e.g., the greatest equatorward advance of Pleistocene glaciation).	","","","","","",""
"glacial rebound","The isostatic adjustment of previously glaciated areas after glacial retreat (e.g., the uplift of Scandinavia after the most recent glaciation.	","","","","","",""
"glacier","A mass of land ice that is formed by the cumulative recrystallization of firn. A glacier flows slowly from an accumulation area to an ablation area. Some well-known glaciers are: the Zermatt,  Stechelberg, Grinelwald,  and Les Diablerets in Switzerland; the Nigards, Gaupne,"," Fanarak, and Lom in Norway; the Wright"," Taylor and Wilson Piedmont glaciers in Antarctica; Grinnell glacier in Glacier National Park,  Montana, U.S., and the Teton glacier in Teton National Park, Wyoming, U.S.	","","","",""
"glaclolacustrine","Used to describe sediments deposited in a lake in contact with or receiving most of its water and sediments from a glacier, and also landforms resulting from such deposition.	","","","","","",""
"graben","A relatively depressed crustal unit bounded by faults on its long sides. Grabens may range in scale from meters to many kilometers. Rift valley becomes the preferred term when dimensions approach the thickness of the brittle crust (>30 km).","","","","","",""
"great earthquake","An earthquake having a magnitude of 8 or greater on the Richter scale.","","","","","",""
"greenhouse or icehouse","Useful polarized concepts of the state of the planet. "Greenhouse" collects together global warmth, flattened thermal gradients, increased humidity, high sea level and reduced vigor of oceanic circulation with a tendency toward enhanced anoxia in some parts of the ocean. "Icehouse" is essentially the reverse state in that assemblage of generalizations. It is important to add not-so-icecap-prone and icecap-prone, respectively, without implying necessarily that the world was ever ice-free.	","","","","","",""
"grenz","A soil horizo,  frequently marked by a bed of clay that results from a temporary halt in the accumulation of vegetal material.	","","","","","",""
"groundwater","The supply of fresh water found beneath the surface of the Earth (usually in aquifers) that often supplies wells and springs.	","","","","","",""
"harmonic tremor","A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault. The seismic signal is characterized by a nearly monotonic sinusoidal appearance and low frequency (0.5-5 Hz).","","","","","",""
"heat sink","That portion of a thermodynamic system that absorbs unused heat.	","","","","","",""
"hertz","The unit of frequency equal to one cycle per second.	","","","","","",""
"historical biogeography","Discipline that deals with the ever-changing distribution of organisms, marine and non-marine, over the earth's surface through geologic time; i.e., the historical counterpart of present-day biogeography, which discusses the modern distribution of all living organisms.	","","","","","",""
"Holocene","The most recent epoch of the Quaternary period"," covering approximately the last 10,000 years.	","","","","",""
"homopycnal","Describes water entering a lake that has the same density as the lake water.	","","","","","",""
"hotspot","Name commonly given to the surface expression (uplift, volcanism, high-heat flow) of an upwelling mantle plume.	","","","","","",""
"humic acids","Suite of organic acids (RCOOH) of indefinite composition and molecular structure produced by decomposition and condensation reactions. Operationally defined as being solubilized from peat, humus, soil, sediment or other material by dilute-alkali extraction.	","","","","","",""
"hydrosphere","The aqueous envelope of the Earth, including the oceans, freshwater lakes, rivers, saline lakes and inland seas, soil moisture and vadose water, groundwaters and atmospheric vapor.	","","","","","",""
"hydrothermal activity","Interaction of an aqueous solution with rock at high temperature and pressure, leading to alteration of the rock and dissolution of some of the rock minerals and formation of others.	","","","","","",""
"hyperpycnal","Describes water that is denser than the water in the lake it enters.	","","","","","",""
"hypocenter","The calculated location of the focus of an earthquake.","","","","","",""
"ice age","A glacial epoch or time of extensive glacial activity. Also,  as Ice Age, which refers to the latest glacial epoch, the Pleistocene Epoch. Periods characterized by very low temperature worldwide and advancing glaciers.	","","","","","",""
"ice core","Deep drill into the earth's permanent glaciers revealing the history of the atmospheric gas and dust content, enabling deduction of former atmospheric temperatures.	","","","","","",""
"ice sheet","A glacier of considerable thickness and more than 50,000 sq. km in area. It forms a continuous cover of ice and snow over a land surface. An ice sheet is not confined by the underlying topography but spreads outward in all directions. During the Pleistocene Epoch"," ice sheets covered large parts of North America and northern Europe but they are now confined to polar regions (e.g., Greenland and Antarctica).  Also called continental glacier.	","","","","",""
"ice shelf","A sheet of very thick ice with a level or gently undulating surface. It is attached to the land on one side, but most of it is floating. On the seaward side it is bounded by a steep cliff (ice front) two to 50 m or more above sea level. Ice shelves have formed along polar coasts (e.g., Antarctica and Greenland); they are very wide, with some extending several hundreds of kilometers toward the sea from the coastline. They increase in size from annual snow accumulation and seaward extension of land glaciers. They decrease in size from warming,"," melting and calving.	","","","","",""
"ice stream","Large, fast glaciers embedded in slow-moving ice. Flow velocities are a few hundred meters to kilometers per year.	","","","","","",""
"iceberg","A floating ice mass up to 100 km long and 200-300 m thick that has broken off (calved) from ice shelves, glaciers or coastal ice cliffs into the ocean.	","","","","","",""
"impact crater","Circular, rimmed depression produced by the impact of a solid body traveling at more than a few kilometers per second.	","","","","","",""
"impedance","Apparent resistance to flow of alternating current; it is analagous to electrical resistance in direct current method and is a complex quantity. For plane wave electromagnetic sources. the impedance of the ground is equal to the ratio of the orthogonal horizontal electric and magnetic fields.	","","","","","",""
"induction","Process by which a body becomes magnetized or electrified by merely placing it in a magnetic or electric field.  Also refers to the process under which electric currents are initiated in a conductor by merely placing it in an electromagnetic field.	","","","","","",""
"infrared radiation","Electromagnetic radiation lying in the wavelength interval from 0.7 microns to 1000 microns. Its lower limit is bounded by visible radiation and its upper limit is bounded by microwave radiation. Most of the energy emitted by the Earth and its atmosphere is at infrared wavelengths. Infrared radiation is generated almost entirely by large-scale intramolecular processes. The tri-atomic gases, such as water vapor, carbon dioxide, and ozone absorb infrared radiation and play important roles in the propagation of infrared radiation in the atmosphere. Abbreviated IR; also called longwave radiation.	","","","","","",""
"integrated time scale (Eocene)","The Eocene rock record initially was and still is recognized by its characteristic fossils; fossils are cross-checked against the geomagnetic reversal scale. But rates of change and other quantifications demand numerical calibration -- time in millions of years -- which is founded ultimately in isotopic decay in the potassium-argon and other decay series. The geomagnetic scale consists of chrons, of which Chron C24 and Chron C19 are important here. Some fossil successions have their own names; thus, e.g., the Wasatchian  North American "land mammal age."","","","","","",""
"intensity","A measure of the effects of an earthquake at a particular place on humans, structures and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point.","","","","","",""
"ion","An atom or molecule that has an electric charge.	","","","","","",""
"isoseismal line","A line connecting points on the Earth's surface at which earthquake intensity is the same. It is usually a closed curve around the epicenter.","","","","","",""
"isostasy","The vertical readjustment of the surface of the earth due to the addition or removal of weight. Commonly associated with the advance and retreat of glacial ice.","","","","","",""
"isostatic adjustment","The process whereby lateral transport at the Earth's surface from erosion or deposition is compensated for by movements in a subcrustal layer to maintain equilibrium among units of varying masses and densities. Also called isostatic compensation.	","","","","","",""
"isotherm","A line on a chart that connects all points of equal or constant temperature.	","","","","","",""
"isotope","One of two or more atoms that have the same atomic number (i.e., the same number of protons in their nuclei) but have different mass numbers. Forms of the same element that have the same number of protons but differing numbers of neutrons. This mass difference gives rise to slightly differing chemical properties for each isotope. Of interest here are the stble isotopes of carbon: carbon-12 (or 12C) and carbon-13 (or 13C), which are stable over time"," and carbon-14, which is a radioisotope and decays with time to an isotope of nitrogen.	","","","","",""
"isotopes in paleoceanography","Stable isotopes of some light elements occuring in nature in given ratios fractionated by the biosphere and thereby providing informative signals about the latter. Most useful ratios are 16O/18O; 12C/13C. Marine organisms biomineralize using oxygen in their calcite, etc.,  and in doing so record the temperature of the reservoir (i.e., the ocean) in the 16O/18O ratio, which is expressed as delta-18O, a departure from an agreed standard. But evaporation removes a bit more 16O -- a lighter fraction -- from the reservoir. That does not matter at geological time scales until that very light fraction is locked up as an icecap, whereupon the reservoir is distorted in the heavier direction. Many believe that that ice effect is not a great distorter in the Eocene; it becomes assertive in the Oligocene. Carbon behaves differently. Photosynthesis removes 12C preferentially,  so that calcite skeletons are enriched in 13C. Thus high productivity is signaled as an increased difference between the planktonic record (from the photic zone) and the benthic record (where the light fraction is returned to the system). But when both signals go positive through time that shift signals a removal of organically fixed carbon from the reservoir altogether -- into oceanic sediments,  becoming a potential source of oil,  or into increased terrestrial plant biomass and thence into coals.	","","","","","",""
"j”kulhlaup","Sudden, often catastrophic, flood discharge resulting from the failure of part of a glacier-ice or glacial-drift barrier that forms part of the basin of a glacial lake.	","","","","","",""
"kerogen","Solid bituminous mineraloid substance in oil shales.	","","","","","",""
"komatilte","Volcanic rock containing more than 18 perceny by weight MgO.	","","","","","",""
"Krakatoa","Active volcano,  elevation 813 m (2667 ft), which forms an island in West Indonesia","between Java and Sumatra (6.102 S,  105.423 E). Its eruption in 1883, which was one of the most violent in modern times, scattered debris and darkened skies over vast areas. Additional eruptions occurred in late 1927 and in the 1960s.
","","","","",""
"lahar	","A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow. Also known as a "glowing avalanche."","","","","","",""
"landslide","An abrupt movement of soil and bedrock downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can cause tsunamis.","","","","","",""
"langley","The unit of solar energy relating to the amount which reaches a specific area of the earth's surface.","","","","","",""
"lapilli","Literally "little stones;" round to angular rock fragments measuring 1/10 inch to 2 1/2 inches in diameter, which may be ejected in either a solid or molten state.","","","","","",""
"large-ion lithophile elements","Elements with an affinity for oxygen-containing compounds, particularly silicate minerals, and possessing ionic radii larger than most common rock-forming elements. Examples include K, Cs, Th, U, La, Ba.	","","","","","",""
"lateritization","Soil-forming process typical of warm humid climates where in mature landscapes, primary minerals are essentially completely weathered away to form red soils (laterites) of high aggregate stability composed primarily of sesquioxides (oxides and hydroxides of iron and aluminum).	","","","","","",""
"latitude","The location of a point north or south of the equator. Latitude is shown on a map or globe as east-west lines parallel to the equator.","","","","","",""
"lava","Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.","","","","","",""
"lava flow","An outpouring of lava onto the land surface from a vent or fissure. Also a solidified tongue-like or shee- like body formed by outpouring lava.","","","","","",""
"lava tube","A tunnel formed when the surface of a lava flow cools and solidifies, while the still-molten interior flows through and drains away.","","","","","",""
"leaking mode","A surface seismic wave which is imperfectly trapped so that its energy leaks or escapes across a layer boundary, causing some attenuation or loss of energy.","","","","","",""
"Lg wave	","A surface wave which travels through the continental crust.","","","","","",""
"limb","The edge of a celestial body as it appears to an observer; the edge of the sun's disk, for example.	","","","","","",""
"liquefaction","The process in which a solid (soil) takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress.","","","","","",""
"liquid of descent","In a graph of chemical composition, a line connecting magmas related by differentiation to a common parent. The chemical expression of magmatic evolution.	","","","","","",""
"lithosphere","The component of the Earth's surface comprising the rock, soil and sediments. It is a relatively passive component of the climate system"," and its physical characteristics are treated as fixed elements in the determination of climate. Semirigid upper platelike layer of the earth. Its thickness is near zero at oceanic ridge axes, but it increases with age to about 100 km. Oceanic lithosphere consists of a crustal layer ~5 km thick of basaltic composition underlain by mantle rocks. Continental lithosphere consists of a layer of crustal rock 30-60 km thick of low density and variable composition underlain by mantle rocks.
","","","","",""
"loess","A buff-colored, wind-blown deposit of fine silt,  which is frequently exposed in bluffs with steep faces. The thickness can range from 6 to 30 m. The loess of the United States and Europe is thought to be the fine materials first transported and deposited by the waters of melting ice sheets during the glacial period. It was later blown considerable distances with  in some cases"," deposition in lakes. The origin of Asiatic loess"," however"," is apparently wind-blown dust from central Asian deserts.	","","",""
"long-period event","Discrete events with nearly monotonic,  low-frequency (1-5 Hz) waveforms that resonate for many cycles.	","","","","","",""
"longitude","The location of a point east or west of the prime meridian. Longitude is shown on a map or globe as north-south lines left and right of the prime meridian, which passes through Greenwich, England.","","","","","",""
"Love wave","A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation (travel). It is named after A.E.H. Love, the English mathematician who discovered it.","","","","","",""
"low-frequency event","General term for discrete events with either enriched low frequencies or deficient high frequencies compared to typical earthquakes.	","","","","","",""
"low-velocity zone","Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below.","","","","","",""
"macroturbulence	Flood flows of great depth and high velocity are characterized by special turbulent flow phenomena that are exceedingly powerful. Macroturbulence phenomena include powerful upward vortices called kolks"," intences shocks resulting from sudden pressure change known as cavitation"," and powerful roller vortices oriented parallel to flow. These features provide enormous shear stresses and stream power to flood flows.	","","","","",""
"magma","Naturally occurring liquid rock often containing volatiles, such as water or carbon dioxide. Igneous rocks form by solidification of magma, usually by freezing. Molten silicate rock with dissolved gases. If erupted and degassed it is termed lava.	","","","","","",""
"magma chamber","The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.","","","","","",""
"magnetic diffusivity","Constitutive pressure associated with the diffusive manner in which magnetic field lines slip through electrically conducting fluid as a result of the action of dissipation caused by finite electrical reisitance.	","","","","","",""
"magnetic field lines","Mathematical construction: a family of curves in space giving at each point the direction (by the tangent of the line) and the strength (by the local density of lines) of a magnetic field.	","","","","","",""
"magnetic jerk","Change in the slope of the geomagnetic secular variation. The change occurs within a few years and is global in extent.	","","","","","",""
"magnitude","A measure of the strength of an earthquake or strain energy released by it as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example,  from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words"," a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake -- or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake! There is no beginning nor end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude","-1.0 event releases about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are generally not felt by humans.","","","",""
"major earthquake","An earthquake having a magnitude of 7 to 7.99 on the Richter scale.","","","","","",""
"mantle","The layer of rock that lies between the crust and the outer core of the Earth. It is approximately 2900 kilometers thick and is the largest of the Earth's major layers.","","","","","",""
"mass balance","The application of the principle of the conservation of matter. For example,  the mass of a glacier is not destroyed or created; the mass of a glacier and all its constitutive components remains the same despite alterations in their physical states. The mass balance of a glacier is calculated with the input/output relationships of ice, firn and snow,  usually measured in water equivalent. Output includes all ablative processes of surface melting basal melting"," evaporation"," wind deflation"," calving"," and internal melting. Input includes direct precipitation"," avalanching"," and the growth of superimposed ice.
"
"massive sulfide ore deposit","Occurrence of a concentrated mass of sulfide minerals such as pyrite,  sphalerite  or chalcopyrite in one place,  as opposed to their being disseminated or occurring in veins.	","","","","","",""
"Mauna Loa","An intermittently active volcano, elevation 4170 m (13,680 ft) on the island of Hawaii (19.475 N, 155.608 W); the last eruption was in 1984. Also see Mauna Loa record.	","","","","","",""
"mean sea level","The average height of the sea surface"," based upon hourly observation of the tide height on the open coast or in adjacent water that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stages of the tide over a nineteen year period. Mean sea level, commonly abbreviated MSL and referred to simply as "sea level," serves as the reference surface for all  altitudes in upper atmospheric studies.	","","","","",""
"metamorphic","From the Greek "meta" (change) and "morph" (form). Commonly occurs to rocks which are subjected to increased heat and/or pressure. Also applies to the conversion of snow into glacial ice.","","","","","",""
"microearthquake","An earthquake having a magnitude of 2 or less on the Richter scale.","","","","","",""
"micromorphology","Texture of a soil as viewed in petrographic thin section.	","","","","","",""
"microseism","A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents.","","","","","",""
"Milankovitch theory","An astronomical theory formulated by the Yugoslav mathematician Milutin Milankovitch that associates climate change with fluctuations in the seasonal and geographic distribution of insolation determined by periodic variation of the Earth's eccentricity and obliquity and the longitude of perihelion.	","","","","","",""
"mineral","A naturally occurring, inorganic,  crystalline solid with a definite internal structure and chemical composition.","","","","","",""
"mineral resource analysis","Probabilistic modeling of occurrences and sizes of undiscovered deposits for mineral and energy resources evaluation and exploration.	","","","","","",""
"model age","Age of a rock or mineral that is determined by isotopic methods in which the initial isotopic ratio of the sample assumed.	","","","","","",""
"modeling","An investigative technique that uses a mathematical or physical representation of a system or theory that accounts for all or some of its known properties. Models are often used to test the effects of changes of system components on the overall performance of the system.	","","","","","",""
"Modified Mercalli intensity scale","Proposed in 1902 by Giuseppi Mercalli of Italy; updated in 1931 by H.O. Wood and F. Neumann as Modified Mercalli; further revised in 1956 by C. F. Richter and presented here in abridged form. Earthquake intensity on the MM scale is given as Roman numerals to prevent confusion with Richter magnitudes and refers to the earthquakes maximum intensity.","","","","","",""
"Mohorovicic discontinuity","The boundary surface or sharp seismic-velocity discontinuity (pronounced Mo-ho-ro-vi-chich) that separates the Earth's crust from the underlying mantle. Named for Andrija Mohorovicic, the Croatian seismologist who first suggested its existence.","","","","","",""
"mudflow","A flow of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.","","","","","",""
"multivariate analysis","Simultaneous statistical analysis with respect to more than two variables; includes regression analysis for expressing a dependent variable in terms of explanatory variables with diagnostics for influence of observations and variables.	","","","","","",""
"norite","An igneous rock with mafic composition.","","","","","",""
"normal modes of vibration","Natural resonances on the coupled periodic displacements of atoms in a material about their equilibrium positions.	","","","","","",""
"normal polarity","Geomagnetic polarity that is the same as the present polarity.	","","","","","",""
"nu‚e ardente","A French term applied to a highly heated mass of gas-charged ash which is expelled with explosive force down the mountainside; a significant volcanic hazard.","","","","","",""
"obsidian","A black or dark-colored volcanic glass"," usually of rhyolitic (felsic) composition.","","","","",""
"oceanic magnetic anomalies","Systematic fluctuations in magnetic field intensity that are a result of the juxtaposition of ocean floor formed (and magnetized) during intervals of normal and reversed geomagnetic polarity.	","","","","","",""
"olivine","Mineral of (MgFe)2SiO4 composition. It is the main component of the upper mantle.	","","","","","",""
"open hydrochemical system","Portion in which the mass of a chemical element is not conserved locally and migrates across the boundaries of the system in solution as a solute or as a suspended mineral particle.	","","","","","",""
"ophiolite","Assemblage of rock units including pillow basalts, diabase sheeted dikes,  gabbros and serpentized periodites lying uncomformably on continental lithosphere, which likely represent sections of oceanic lithosphere accreted to the continent during plate collision. Body of rock composed of some or all of the following principal rock units: basaltic lava; sheeted diabase dikes; cumulate gabbro,  norite,  pyroxenite,  and periodite; metamorphic-textured herzolite,","harzburgite and dunite. Layered suite of basic and ultrabasic silicate rocks consisting of pillow basalts,  basaltic dikes,  gabbros and periodites that are thought to represent segments of oceanic lithosphere that have been thrust onto a convergent plate margins.","","","","",""
"order of magnitude","Ten times; thus an increase of two orders of magnitude is 10 x 10 or 100 times.	","","","","","",""
"orogenic zone","Linear region of the crust, where the lithosphere is relatively mobile and deformed into high-standing mountain belts. Most commonly forms in association with subduction zones or continental collisions.	","","","","","",""
"P wave","Primary,  longitudinal, irrotational, push,  pressure,  dilatational, compressional"," or push-pull wave. P waves are the fastest body waves and arrive at stations before the S waves"," or secondary waves. The waves carry energy through the Earth as longitudinal waves,","moving particles in the same line as the direction of the wave. P waves can travel through all layers of the Earth. P waves are generally felt by humans as a bang or thump.","","",""
"pahoehoe","Hawaiian term for a fluid volcanic eruption resulting in broad basaltic shield volcanoes. The highly fluid magma flows readily and hardens into ropey forms as it cools.","","","","","",""
"paleocompetence","Competence is a measure of a stream's abiility to entrain sediment particles, measured by the maximum grain size that can be transported. Empirical relationships between grain size and flow velocity or stream power are applied to paleoflood deposits to derive estimates of these same paleoflow parameters.	","","","","","",""
"paleogeography","Working out by means of geologic criteria (chiefly chemically sensitive sedimentary rocks and environmentally sensitive rocks and structures,  such as nonmarine shoreline"," or deep-water types) and structural geologic evidence for the displacement of rock bodies that originally were continuous geophysical criteria (chiefly the evidence of remanent magnetism, which provides data regarding the original latitudinal position of rock bodies,"," although now whether that latitude is north or south) and paleontologic criteria (chiefly the evidence of historical biogeography that helps to establish which rock bodies contain fossils"," suggesting original reproductive communication between same) of the ever-changing geographies of the past.
","","",""
"paleomagnetism","The natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past. Also, the study of these magnetic traces.","","","","","",""
"paleorift","Dead or dormant rift structure with no remaining transient or thermal phenomena (recent or contemporary magmatism or enhanced heat flow).	","","","","","",""
"paleoseismology","Study of prehistoric earthquakes, often by excavation across an active fault to examine evidence of past surface-rupturing events.	","","","","","",""
"partitioning","Process by which a given element distributes itself to different extents in two ohases in equilibrium with each other. The ratio of the concentration of the element in one phase limits and is known as the Nernst partition coefficient.	","","","","","",""
"pegmatold","Name given to a coarse-grained rock in which minerals have a crystalline outline and range from one to several centimeters in size, as in pegmatite, but which lacks the large proportions of quartz and orthoclase that characterize pegmatite.	","","","","","",""
"percolation","The movement of water downward and radially through the subsurface soil layers, usually continuing downward to the groundwater.	","","","","","",""
"period","The time between two successive wave crests.","","","","","",""
"permafrost","Perenially frozen ground that occurs whereever the ground temperatures remain continuously below 0ø C for two or more years.  Discontinuous or patchy permafrost occurs north of the 0ø C mean annual air temperature isotherm; continuous permafrost up to several hundred meters in thickness is widespread in Siberia and northern Alaska and Canada.
","","","","","",""
"perovskite","Structure of the main mineral phase of the lower mantle. (MgFe)SiO3 perovskite represents approximately 40% by volume of the entire earth.	","","","","","",""
"phase","The onset of a displacement or oscillation on a seismogram indicating the arrival of a different type of seismic wave.","","","","","",""
"phenocryst","Large conspicuous crystal set in a fine-grained matrix.	","","","","","",""
"phreatic eruption (explosion)","An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.","","","","","",""
"pisolites","Concretionary spherical nodules formed by transformation of duricrust by accretionary growth and dilation in the uppermost laterite zone.	","","","","","",""
"planetesimal","Bodies in the solar nebul,  formed from accumulation of dust grains. They vary greatly in size from a few microns up to the size of small planets.	","","","","","",""
"planktonic foraminifera","Planktonic protozoans which may form carbonate tests. The tests are preserved in sediments and form an important basis for paleoceanographic reconstructions.	","","","","","",""
"plate","One of the huge sections which make up the Earth's crust. The plates are continuously moving.","","","","","",""
"plate boundary","The place where two or more plates in the Earth's crust meet.","","","","","",""
"plate tectonics","The theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving,","segments or plates.","","","","",""
"plate tectonics","The theory that the earth's crust is broken into many large fragments (plates), which move in relation to one another, shifting continents, forming new ocean crust, and causing volcanic eruptions.","","","","","",""
"Pleistocene","The earlier of the two periods of the Quaternary period,  starting 2 to 3 million years before the present and ending about 10,000 years ago. It was a time of glacial activity. A vast amount of evidence has been collected that indicates dramatic oscillations in climatic conditions of global proportion throughout the Pleistocene. It has been well documented that global ice mass has undergone huge fluctuations on the order of 10E19 kg (equivalent to more than 100 meters of sea level) during the last million years. In addition,  it has been demonstrated that the concentrations of atmospheric CO2 vary by at least 30% through a glacial cycle, with glacial periods characterized by low pCO2 (~190 ppm) and interglacials by high pCO2 (~280 ppm). Ocean temperature and circulation have also undergone glacial-interglacial change.","","","","","",""
"plug","Solidified lava that fills the conduit of a volcano. Plugs (also called volcanic necks) are usually more resistant to erosion than the material making up the surrounding cone"," and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.","","","","",""
"plug dome","The steep-sided,  rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.","","","","","",""
"plume","Volume of contaminated groundwater that occupies a continuous region of an aquifer and emanates from a single source.	","","","","","",""
"pluton","A large igneous intrusion formed at great depth in the crust.","","","","","",""
"podzolization","Soil-forming process typified by formation of humic acids and leaching of acids and sesquioxides. Continuum of soils developed range from peats, podzol soils (upper mineral soil bleached nearly white by organic acids) to podzolic soils where leaching of sesquioxides is insufficient to bleach mineral soil.	","","","","","",""
"polar motion","Path of the earth's spin axis relative to an earth fixed-coordinate system. Movement of the rotation axis relative to geographic (earth-fixed) coordinates, consisting mainly of a slow drift, a 14-month Chandler wobble"," and a 12-month annual wobble.	","","","","",""
"polarity chron","Time interval of constant geomagnetic polarity.	","","","","","",""
"polarity zone","Stratigraphic interval in which the rocks or sediments carry a magnetization indicating formation in a field of constant polarity.	","","","","","",""
"positive feedback","An interaction that amplifies the response of the system in which it is incorporated.	","","","","","",""
"potential energy (gravitational)","The stored energy of a substance. Water has a lot of this if there is an elevation difference. Potential energy can be converted to kinetic energy if the water (or other substance) is allowed to move.","","","","","",""
"potentiometric surface","Imaginary surface defined by the levels to which water will rise in wells that are open at the same elevation. The slope of the potentiometric surface determines the horizontal direction of groundwater flow.	","","","","","",""
"precipitation","Any condensed water falling from the atmosphere to the surface of the earth. Common types include rain, snow, sleet and hail.","","","","","",""
"pressure gradient","The rate of decrease in atmospheric pressure per unit of horizontal distance; used to indicate relative pressures on the earth's surface"," and measured in the direction of most rapid decrease.	","","","","",""
"primary magma","Magma produced by melting a solid precursor. Magma only slightly differentiated in composition from primary magma is called primitive.	","","","","","",""
"primary phase field","Area or volume on a phase diagram depicting crystal-melt equilibria in which one crystalline phase precipitates.	","","","","","",""
"principle of opposition","Graphical technique used to model fractional crystallization of a magma that determines the composition of the extract assemblage.	","","","","","",""
"proxy climate indicators","Dateable evidence of a biological or geological phenomenon whose condition, at least in part"," is attibutable to climatic conditions at the time of its formation. Proxy data are any material that provides an indirect measure of climate and include documentary evidence of crop yields,  harvest dates, glacier movements, tree rings,","varves, glaciers and snow lines, insect remains"," marine microfauna, isotope measurements: 18O"," in ice sheets, 18O, 2H,  and 13C in tree rings; CaCO3 in sediments; and speleothems. There are three main problems in using proxy data: (1) dating,   (2) lag and response time,  and (3) meteorological interpretation. Tree rings,"," pollen deposits from varved lakes, and ice cores are the most promising proxy data sources for reconstructing the climate of the last five millennia because the dating are precise on an annual basis while other proxy data sources may only be precise to +/- 100 years.	",""
"pumice","Light-colored, frothy volcanic rock,","usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.","","","","",""
"pyroclastic","Igneous material erupted explosively as finely pulverized lava (bombs, blocks, pumice, ash"," and hot gases).	","","","","",""
"pyroclastic flow","Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments,  crystals, ash, pumice"," and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed.","","","","",""
"quantitative stratigraphy","Use of computer-based mathematical methods to calibrate stratigraphic models that with a minimum of information provide maximum predictive potency and include formulation of confidence limits.	","","","","","",""
"Quaternary period","The latest period of geologic time, covering the most-recent two million years of the Earth's history. It is divided into two epochs: the Pleistocene -- two million years ago to approximately 10,000 years ago -- and the Holocene -- the period from approximately 10,000 years ago to the present. The Quaternary period is the artificial division of time separating prehuman and human periods. It contains five ice ages and four interglaqcial ages,  and temperature indicators seem to show sharp abrupt changes by several degrees.	","","","","","",""
"rayleigh wave","A type of surface wave having a retrograde"," elliptical motion at the Earth's surface"," similar to the waves caused when a stone is dropped into a pond. These are the slowest"," but often the largest and most destructive"," of the wave types caused by an earthquake. They are usually felt as a rolling or rocking motion and in the case of major earthquakes"," can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence.",""
"recharge","The process by which water is added to a reservoir or zone of saturation, often by runoff or percolation from the soil surface.	","","","","","",""
"recharge area","Region in which there is a net addition of water to the groundwater system (recharge) as a result of infiltration from surface water bodies or an excess of precipitation over, evapotranspiration and runoff.	","","","","","",""
"recurrence Interval","The approximate length of time between earthquakes in a specific seismically active area.","","","","","",""
"reflectivity","The ratio of the energy carried by a wave that is reflected from a surface to the energy of a wave incident on the surface.
","","","","","",""
"regolith reduction","Diminishment in volume and mass of parent material to a smaller volume and mass of weathered soil residue as mobile constituents are carried away, in solution or by aeolian transport.	","","","","","",""
"reservoir","Any natural or artificial holding area used to store, regulate, or control a substance.	","","","","","",""
"residual enrichment","Increase in the enrichment factor of a relatively immobile element by virtue of hydrochemical leaching of mobile species.	","","","","","",""
"reverse fault","General term for a fault dipping between 30ø and vertical, and with the hanging wall moving up with respect to the foot wall.	","","","","","",""
"reverse weathering","Collection of processes that lead to return of protons consumed in the process of chemical weathering to earth's surface system.	","","","","","",""
"reversed polarity","Geomagnetic polarity that is opposite to the present polarity.	","","","","","",""
"rheology","Science of the deformation and flow of matter.	","","","","","",""
"rhyolite","Volcanic rock (or lava) that characteristically is light in color, contains 69 percent silica or more, and is rich in potassium and sodium. It is fine grained, which although different in texture has the same composition as granite.","","","","","",""
"Richter scale","The system used to measure the strength of an earthquake. Developed by Charles Richter in 1935 as a means of categorizing local earthquakes. It is a collection of mathematical formulas; it is not a physical device.","","","","","",""
"rift system","The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their land counterparts, such as the East African Rift.","","","","","",""
"ring current","Electrical current at about three earth radii from the center of the earth. The current results from charged particles trapped in the geomagnetic field.	","","","","","",""
"ring of fire","The regions of mountain-building earthquakes and volcanoes that surround the Pacific basin.","","","","","",""
"rupture zone","The area of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquake","the rupture zone may extend several hundred kilometers in length and tens of kilometers in width.","","","","",""
"S wave	","Shear,  secondary, rotational, tangential,"," equivoluminal,  distortional, transverse"," or shake wave. These waves carry energy through the Earth in very complex patterns of transverse (crosswise) waves. These waves move more slowly than P waves, but in an earthquake they are usually bigger. S waves cannot travel through the outer core because these waves cannot exist in fluids, such as air,","water or molten rock.","","",""
"S-wave","Secondary wave, a shear or transverse elastic wave, usually the most prominent wave on a seismogram; travels more slowly than P-waves and thus arrives later on seismograms.	","","","","","",""
"salt water intrusion","The invasion of fresh surface or groundwater by salt water.	","","","","","",""
"saprolite","Fine-grained weathering zone with microscopic or submicroscopic porosity occurring immediately above parent material bedrock. Saprolite is not contaminated by invasion of detritus from above and may or may not be isovolumetric.	","","","","","",""
"sapropel","Lithified organic-rich marine sediment deposited under anoxic conditions. If the organic content is very high, the sediments lithify to become black shale; if dilution by clays or other materials occurs, the sediments lithify to become marlstone.	","","","","","",""
"ScS","Seismic phase consisting of a shear wave traveling down through the mantle that is reflected at the core-mantle boundary and returns to the earth's surface as a shear wave. ScSn denotes a multiple ScS wave that is further reflected n-1 times at the earth's free surface.	","","","","","",""
"sea aerosol","Colloidal-size particles of substances derived from the sea surface.	","","","","","",""
"seafloor spreading","The mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as tectonic plates separate.","","","","","",""
"secular variation","That portion of the temporal variation of the geomagnetic field due to time-varying external current systems are excluded form this definition.	","","","","","",""
"sediment","Rock debris commonly produced by mechanical or chemical weathering processes.","","","","","",""
"sedimentary petrology","Study of the composition, characteristics"," and origin of sediments and sedimentary rocks.	","","","","",""
"seiche","A free or standing wave oscillation of the surface of water in an enclosed basin that is initiated by local atmospheric changes, tidal currents or earthquakes. Similar to water sloshing in a bathtub.","","","","","",""
"seismic","Pertaining to earthquake activity.	","","","","","",""
"seismic belt","An elongated earthquake zone, for example,"," circum-Pacific, Mediterranean, Rocky Mountain. About 60% of the world's earthquakes occur in the circum-Pacific seismic belt.","","","","",""
"seismic constant","In building codes dealing with earthquake hazards, an arbitrarily-set acceleration value (in units of gravity) that a building must withstand.","","","","","",""
"seismic discontinuity","Regionally extensive, thin zones in the crust or mantle where seismic velocities change abruptly. May be the result of changes in composition,  mineral phases or structural character.	","","","","","",""
"seismic sea wave","A tsunami generated by an undersea earthquake.","","","","","",""
"seismic zone","A region in which earthquakes are known to occur.","","","","","",""
"seismicity","Earthquake activity.","","","","","",""
"seismogram","A written record of an earthquake recorded by a seismograph.","","","","","",""
"seismograph","An instrument that records seismic waves; that is vibrations of the earth. Used to record and measure earthquakes.","","","","","",""
"sferics","Natural atmospheric fluctuations of the electromagnetic fields, caused mainly by thunderstorm activity in the range of 1-10E5 Hz.	","","","","","",""
"shearing","The motion of surfaces sliding past one another.","","","","","",""
"shield","Tectonically stable, low-lying Precambrian nucleus of continents. Commonly contains the metamorphosed roots of ancient orogenic activity.	","","","","","",""
"shield volcano","A gently sloping volcano in the shape of a flattened dome built almost exclusively of mafic lava flows. The Hawaiian Islands are a good example.","","","","","",""
"shock metamorphism","Characteristic changes in rock mineralogy caused by the compression of a passing shock wave.	","","","","","",""
"shock wave","Moving wave of strong compression that usually travels faster than sound in the medium.	","","","","","",""
"signal-to-noise ratio","A quantitative measure of the statistical detectability  of a signal, expressed as a ratio of the magnitude of the signal relative to the variability. For first detection of a CO2-induced climate change, the model signal is the mean change or anomaly in some climatic variable,","usually surface air temperatures, attributed by a numerical model to increased concentrations of carbon dioxide. Observed noise is the standards deviation or natural variability computed from observations of that variable and agjusted for sample size,  autocorrelation and time averaging.	","","","","",""
"skin depth","Measure of the effective depth of penetration of electromagnetic fields. It is defined as the depth at which the amplitude of an electromagnetic field drops to 37% of its surface value.	","","","","","",""
"snowline","The lower limit of any year's permanent snowfall. Separates the Zone of Accumulation from the Zone of Ablation.","","","","","",""
"soft mode","Molecular vibrational mode for which the atomic displacements track the structural changes followed during a displacive phase transition. The frequency of the soft mode goes to zero at the transition temperature or pressure.	","","","","","",""
"soil","In a geologic sense of the term,  a weathered horizon at the earth's surface. During periods of landscape stability,  various physical and chemical processes modify the upper few meters of the surface. Typically,  new mineral assemblages such as clays are introduced,"," as is organic matter, and calcium carbonate if the climate is dry. Once a soil is buried by other sediments, it is then referred to as a paleosol.	","","","","",""
"soil carbonate","In regions that receive about 1.3 m of rain or less,  calcium carbonate will often accumulate in the lower portion of a soil profile. In fine-grained soils, such as under the prairies of the mid-western United States,  the carbonate often takes the form of discrete nodules up to several centimeters in diameter. In gravelly soils such as are common in the western U.S.,  carbonate will begin by coating clasts,  and with time it will entirely plug soil voids, forming what is commonly called a caliche or more formally a calcic horizon.	","","","","","",""
"soil profile","Vertical section of the soil through all of its distinct layers (horizons) produced by the soil-forming process and extending into the parent material.	","","","","","",""
"solid state","In metamorphism, indicates the change of mineral identity without melting. All ion migration occurs while the rock (or pre-glacial ice) is still solid.","","","","","",""
"solute","Organic or inorganic species that is dissolved in groundwater.	","","","","","",""
"Soret effect thermal diffusivity)","Solute flux due to a temperature gradient.	","","","","","",""
"spasmodic tremor","Continuous seismic signal with pulsating high-frequency appearance; can be composed of discrete earthquakes occurring closely spaced in time or can be "phaseless."	","","","","","",""
"speleothem","A mineral deposit formed in a cave by the action of water.	","","","","","",""
"spines","Horn-like projections formed upon a lava dome.","","","","","",""
"spontaneous magnetization","Magnetization resulting from cooperative behavior of unpaired spins within a crystal lattice,  creating a magnetic moment that is independent of the external field.	","","","","","",""
"spread","The layout of seismometer or geophone groups from which data from a single shot (the explosive charge) are recorded simultaneously.","","","","","",""
"spreading center","An elongated region where two plates are being pulled away from each other. New crust is formed as molten rock is forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.","","","","","",""
"spring","A surface flow of groundwater which occurs any time the water table intersects the surface.","","","","","",""
"stochastic inversion","Method for computing Gaussian coefficients (or equivalent parameters) that is based on a priori estimates of parameter and noise covariances.	","","","","","",""
"strain","Change in a volume of a system induced by deformation; it is positive for dilation and negative for contraction and is distinct from regolith or volume reduction, which may be an isovolumetric removal of soluble constituents in solution without deformation.	","","","","","",""
"stratification","Separating into layers.	","","","","","",""
"stratovolcano","A volcano composed of both lava flows and pyroclastic material. Also called "Composite" volcanoes. Common at convergent boundaries. Excellent examples in the U.S. include Mt. St. Helens, Mt. Shasta,  and the other peaks of the Cascade Range of California,  Oregon"," and Washington.","","","","",""
"stress probing","Measurement of in situ stresses in a dedicated small-diamter hole drilled ahead of the primary drill bit.	","","","","","",""
"striations (glacial)","Grooves eroded into bedrock by rock debris frozen into the base of a glacier.","","","","","",""
"strike-slip fault","General term for a vertical fault with horizontal displacement vector producing lateral relative motion of the rock on either side.	","","","","","",""
"subduction","The process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the Earth's mantle.","","","","","",""
"subduction zone","An elongated region along which a plate descends relative to another plate, for example,","the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench.","","","","",""
"sublimation","The direct change from the solid to the vapor phase (without passing through the liquid phase). Commonly occurs in ice and snow fields on sunny days above the snowline.","","","","","",""
"submicroscopic","Not resolvable in a microscope.	","","","","","",""
"subsolidus convection","Convection via solid-state deformational processes.	","","","","","",""
"surface of the earth","The value given is the depth below the surface of the mean spheroid. The mean spheroid is a uniform approximation to the true shape of the Earth. No adjustment is made to the depth due to any differences between the true Earth and the mean spheroid. For example, the minimum depth that will be given is 0 kilometers  even though a quake directly under Mount Everest (elevation 8848 meters) could legitimately have a depth of -6 kilometers and still be 2 kilometers underground. On the other hand, a depth of 10 kilometers would actually be more than 1 kilometer above the ocean floor of Challenger Deep (elevation -11,033 meters) in the Marianas Trench of the Pacific Ocean.","","","","","",""
"surface waves","Waves that move over the surface of the Earth. Rayleigh waves and Love waves are surface waves.","","","","","",""
"suspended load","Part of the total stream load that is carried in suspension free from contact with the stream bed; it consists mainly of clay, silt,  and fine sand.	","","","","","",""
"suture zone","Lineation in continental lithosphere marking the location of an earlier plate collision that led to the amalgamation of formnerly separate continental masses.	","","","","","",""
"T-phase","Tertiary wave, a late-arriving short-period phase often recorded at stations of island or coastal regions, containing energy that travels through a layer of water in the ocean known as the SOFAR channel. Can be caused by earthquakes or submarine volcanic eruptions.	","","","","","",""
"tectonic","Pertaining to the deformation of the earth's crust.	","","","","","",""
"teleseism","An earthquake that is distant (usually more than 20 degrees) from the recording station.","","","","","",""
"telluric currents","Natural electric currents that flow in the ground.	","","","","","",""
"TEM","Transient Electromagnetic Method in which the primary energizing field is a repetitive pulse.	","","","","","",""
"tephra","Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.","","","","","",""
"thermophile","Solute diffuses into warmer bottom waters.	","","","","","",""
"tholelite","Relatively silica-rich and alkali-poor basaltic rock containing intermediate to calcic, plagioclase,  clinopyroxene,  and low-Ca pyroxene.	","","","","","",""
"thrust fault","General term for a fault dipping between horizontal and 30ø and with the hanging wall moving up with respect to the foot wall.	","","","","","",""
"till (glacial)","General term for material deposited by a glacier. See also moraine.","","","","","",""
"transcurrent fault","One class of vertical faults with a horizontal displacement vector that does not link two or more plate tectonic scale boundaries.	","","","","","",""
"transform fault","A class of vertical faults with a horizontal displacement vector that does not link two or more plate tectonic scale boundaries.	","","","","","",""
"translocation","Physical or chemical migration of suspended particles or aqueous solutes by moving water in a hydrochemical system or by animals and plants.	","","","","","",""
"translocation crossover","Soil depth at which mass accumulation of an","element by downward translocation changes from positive to zero and then to negative,  thereby separating an upper soil system dominated by invasion of foreign detrital minerals from a lower, less contaminated, soil system referred to as saprolite.	","","","","",""
"travel time","The time required for a wave train to travel from its source to a point of observation.","","","","","",""
"tsunami","Literally "harbor wave" in Japanese. Tsunamis are great sea waves produced by a submarine earthquake, volcanic eruption or large landslide. Tsunamis can cause great damage due to flooding of low coastal areas.","","","","","",""
"tuff","Rock formed of pyroclastic material.","","","","","",""
"turam","Electromagnetic survey method in which the energizing source is a long,  grounded,  insulated cable or a large,  rectangular,  horizontal loop excited by one or more frequencies in the range of 100-1000 Hz.	","","","","","",""
"unconsolidated","Loosely arranged,  not cemented together,"," so particles separate easily.","","","","",""
"unsaturated zone","Zone between the land surface and the water table in which fluid pressures are less than atmospheric.	","","","","","",""
"varve","A layer of sediment deposited in lakes during one year. Each layer consists of two parts, which are deposited at different seasons and which differ in color and texture; thus the layers can be counted and measured. In a complete series the number of layers gives the date on which the ground was vacated by retreating ice.	","","","","","",""
"vent","The opening at the earth's surface through which volcanic materials reach the surface.","","","","","",""
"vesicular basalt","Holes and other openings in basaltic flow which are the result of trapped gas bubbles. Vesicles are often filled at a later date with a wide variety of materials, including quartz,  agate, zeolites, and many other minerals.","","","","","",""
"viscosity","A measure of resistance to flow in a liquid (molasses in January has high viscosity while molasses in August has lower viscosity).","","","","","",""
"volatiles","Elements that are soluble in magmas at elevated pressure and temperature that exsolve as gas from magmas during ascent and eruption at the earth's surface. They include sulfur, water, carbon dioxide,  chlorine and fluorine.	","","","","","",""
"volcanic aerosol","Very small (microns to fraction of micron in diameter) particles or droplets, composed mainly of sulfuric acid and water, produced by the gas-to-particle conversion of sulfur dioxide gas to sulfuric acid in the volcanic eruption plume.	","","","","","",""
"volcanic front","Elongate zone of maximum volcanic activity that defines the main volcanic (or magmatic) arc.	","","","","","",""
"volcanic neck","Solidified lava that fills the conduit of a volcano. Volcanic necks (also called plugs) are usually more resistant to erosion than the material making up the surrounding cone","and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.","","","","",""
"water cycle","The transfer of water between numerous temporary storage reservoirs. These include the ocean,  rivers and streams,  glacial ice, groundwater  and the atmosphere.","","","","","",""
"water table","Surface along which fluid pressure is equal to atmospheric pressure.	","","","","","",""
"weathering (surface)","Weathering includes the processes which mechanically and chemically wear and fragment rock.","","","","","",""
"weights of evidence modeling","Estimation of posterior probability that a hypothesis holds true from its prior probability and a combination of weights associated with presence or absence of features supporting the hypothesis.	","","","","","",""
"wireline logging","Measurement of depth variations (logs) of physical properties using a tool or sonde deployed by means of a conducting cable (wireline).	","","","","","",""
"xenolith","Rock sample with mineralogy foreign to the igneous host in which it occurs. Thought to be a fragment of wall rock from volcanic conduits in the deep lithosphere rapidly transported to the surface during eruptions.	","","","","","",""
"zone of ablation (or wastage)","The area below the snowline where snow melt exceeds snowfall"," and material is lost from a glacier.","","","","",""
"zone of accumulation","The area above the snowline where snowfall exceeds snow melt and material is added to a glacier.","","","","","",""